This invention relates generally to reduction of energy consumption in chemical processes used in the distillation of industrial chemicals, and in particular to distillation systems and methods for the recovery of acetic acid from aqueous solutions. The present invention is particularly suited for the recovery of acetic acid used in the production of terephthalic acid.
Terephthalic acid is useful in a diverse variety of industrial applications and chemical processes. For example, terephthalic acid is a starting material for producing polyesters including plastic and Dacron™ polyester used in textile and container production. Polyethylene terephthalate (PET) is a form of polyester or Mylar™ that is an extremely tough resin and useful in many industrial and consumer applications. Soft drink and water bottles are made from this resin in addition to plastic jars and clamshell packages used in consumer good transport and food distribution. Purified terephthalic acid is a higher grade of terephthalic acid which is used for finer industrial applications.
Terephthalic acid typically is produced by reaction of paraxylene with molecular oxygen in the presence of a catalyst. During the production process, acetic acid is used as a solvent of terephthalic acid. The acetic acid becomes diluted in water during the oxidation in a reactor section of a terephthalic acid plant in the production cycle. A portion of the acetic acid and water containing stream is then sent to a dehydration unit to remove the water generated in the reactor for recycling or waste.
Three different approaches have been employed in the terephthalic acid plants to separate the acetic acid and water so that the acetic acid can be recycled back to the reactor while the water generated by the reaction is sent to the wastewater treatment facility for safe disposal. One approach is by conventional distillation wherein the different boiling point of the components provides for the separation of acetic acid and water. In an azeotropic distillation approach, entrainers are used to form azeotropes with the acetic acid and water providing for a change in energy requirements for processing. Liquid-liquid extraction is a final approach for acetic acid and water separation during the terephthalic acid production.
Distillation has been widely used as a primary unit operation for acetic acid recovery from water. In such processes, one or more towers are utilized to process a number of streams of varying concentration of acetic acid with the purpose of recovering it for further use in the oxidation reactor. The products from the distillation tower are a bottom stream of concentrated acetic acid and an overhead stream that ideally would be pure water to minimize the loss of the valuable acetic acid solvent. A more pure overhead water stream would also reduce the burden on downstream wastewater treatment facilities thereby preventing accidental chemical spills.
However, the distillation of acetic acid and water is not very efficient due to the close-boiling characteristics of the acetic acid/water system. Conventional distillation systems require the use of a high number of theoretical stages, i.e., actual trays, and a high reflux ratio, i.e., high energy consumption, to obtain reasonably low levels of acetic acid, typically in the range of 0.4-0.8 wt % in the overhead distilled water. The distillate is subsequently processed to recover certain organic by-products, and then sent to the wastewater treatment facility where any remaining acetic acid is neutralized and spent.
The use of conventional distillation, therefore, involves high investment cost because of the required large size of equipment and high operating cost because of high steam consumption. Furthermore, the traditional process scheme does not allow one to economically obtain a distillate low in acetic acid concentration. This limitation, in turn, presents operating problems including costs associated with the operation resulting from the acetic acid losses, costs associated with the treatment of the acetic acid in the wastewater, limitations of the capacity of the downsteam wastewater treating facility and environmental problems that are continually increasing because of the ever more rigorous standards for acceptable levels of emission to the environment.
There has been an effort to look for alternative processes to minimize the high operating costs associated with the conventional distillation for the separation of acetic acid and water. Chemical processors and companies have resorted to azeotropic distillation involving the addition of selective alkyl acetate, such as the isobutyl acetate, normal butyl acetate, normal propyl acetate, etc., as an entrainer to the azeotropic dehydration column. The entrainer forms a low boiling azeotrope with water and therefore improves the relative volatility for the separation between the acetic acid containing stream and the alkyl-acetate/water azeotrope. This reduces the energy and theoretical stage requirements for the same separation. Compared to the conventional distillation, an azeotropic distillation approach typically reduces the energy (i.e. steam) consumption by 20-40% at the acetic acid/water dehydration column while giving relatively low acetic acid concentration, 300-800 ppm, in the distilled water. The azeotropic distillation column is generally operated at ambient pressure in the terephthalic acid manufacturing plants in all prior art systems.
Other methods used in terephthalic acid production include the use of liquid-liquid extraction with special extractive agents to recover the acetic acid from the water streams so that the residual concentration is reduced to 0.1 wt % to 2.0 wt % acetic acid. Some of the agents usually are acetates, amines, ketones, phosphine oxides, and mixtures thereof. These agents are used as solvents such that they dissolve one component preferentially, allowing the other component to leave at the top of the extraction column. Once the extraction step is completed, a complicated series of distillation steps is required to recover the acid and to recirculate the solvent back to the extraction column.
Such extraction and azeotropic distillation processes for recovery of acetic acid from aqueous streams are described, for example, by Othmer in U.S. Pat. No. 2,395,010 (1946) and Sasaki et al in U.S. Pat. No. 5,662,780 (1997) and have been applied to the recovery of acetic acid from manufacture of terephthalic acid as described, for example by Ohkoshi et al in Japanese Patent Application JP 244196/95 (1995) and European Patent Application EP 0 764 627.
However, these processes are still energy intensive, and it is desirable to further reduce energy consumption in recovery of acetic acid from such streams.